EP-A-0,422,974 in the name of the Applicant Company describes a process of this type, called a "swinging-type process", intended for the production of gaseous oxygen at a variable flow rate. The second fluid involved is air to be distilled, which is condensed at a variable flow rate.
In this known process, it is easy to show that, in order to keep the supply and delivery flow rates of the distillation unit constant, it is necessary to vary the incoming air flow rate in the same direction as the variations in oxygen demand. If the oxygen is produced under pressure, the air which is condensed in order to vaporize the liquid oxygen is overpressured by an additional booster and, when the oxygen demand varies, it is necessary to vary significantly both the overpressured flow and the flow compressed by the main compressor.
Consequently, in this known process, the compressor, and optionally the booster, are oversized significantly with respect to the nominal oxygen flow rate to be produced. In addition, they work most of the time at considerably lower flow rates compared to their capacities and therefore with downgraded efficiency.
It has also been proposed to store gas to be produced, in gaseous form, in an auxiliary tank or "buffer", at a pressure greater than the production pressure. However, this approach is not satisfactory since it requires very large buffers to be installed in order to satisfy peaks in demand of long duration. In addition, producing all the gas at the buffer pressure is expensive in terms of energy.